The invention generally relates to an apparatus and method for measuring blood pressure, and more particularly to a continuous non-invasive blood pressure measurement and monitoring method and apparatus.
In the case of a hospitalized patient, it has long been desired to be able to provide non-invasive beat-by-beat (continuous) systolic and diastolic blood pressure values. Unfortunately, a practical and reliable solution for obtaining this type of information has yet to be developed.
One technique for providing a continuously measured blood pressure is to insert a saline filled catheter through the patient's vascular system to the point at which the blood pressure measurements are desired. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. As an alternative method, a catheter with the pressure sensor at the tip that directly senses the blood pressure can be inserted into the patient's vascular system. Although both of these techniques have proven effective and continuously monitor a patient's blood pressure, both techniques involve making an incision into the patient's skin and inserting the catheter into the blood vessel. As a consequence, this invasive procedure entails some risk of complication to the patient and is in most cases undesirable.
As yet another alternative, procedures have been developed that favor a tonometric method that does require a blood pressure cuff. However, these methods still require some type of mechanical device that applies pressure to an artery, along with some other type of oscillation (pressure) sensor for the tonometric pressure estimation. Such a device is described in U.S. Pat. No. 6,730,038. Once again, devices of this type have proven unreliable during actual usage.
Other available methods for providing a continuous non-invasive blood pressure determination have tried to use formulas derived from the Bramwell-Hill equation. These methods utilize formulas that rely upon measured arterial pulse wave velocity (PWV) and measured arterial blood volume to determine blood pressure. In each of these methods, the arterial pulse wave velocity (PWV) and the arterial area must be measured. The required PWV measurements are typically obtained by observing the pulse transit time (PTT) between two widely separated sites, such as the heart and the finger tip. The pulse arrival times at the measurement sites, such as the finger tip, are typically determined by pleythysmography or pulse oximetry. One known method for determining PWV is described in U.S. Pat. No. 5,857,975. In this patent, the time of the pressure pulse's origin at the heart is determined from an EKG signal and the arrival time of the pulse is measured at another location on the patient. Based on these measurements, the instantaneous blood pressure is determined. In most systems, initialization data is obtained from a cuff based blood pressure determination and is used along with the time required for the pulse wave to travel between two points to calibrate formulas developed for continuously estimating blood pressure. After the initial calibration, changes in the pulse transit time can be related to changes in the blood pressure. In such schemes, the measurement of the area of the blood flow passageways, the blood flow, and PWV along with the subsequent blood pressure estimate are determined by the use of an assortment of complex adjustment factors.
In any type of non-invasive continuous blood pressure monitoring system, various factors can affect the accuracy of the measurement. For example, changes in the physiological state of the patient can bring about changes in the arterial wall elasticity. In general, changes in the arterial wall elasticity will affect the measured PWV. If the elastic modulus of the arterial wall changes, the same pressure may then need to be associated with a different cross-sectional area and PWV. If the operating point obtained using a blood pressure cuff calibration, then any changes in the arterial elasticity would require re-calibration, and if no re-calibration were performed, errors in the pressure estimation can clearly occur.
Another possible criticism of prior measurement systems concerns the measurement of the arterial blood volume. Specifically, the measurement of the arterial area by using pleythysmography is confounded by the highly elastic nature of the veins in the patient. Since the measurement includes some portion of the venous blood volume, it is difficult to rely upon the measurement to produce a total arterial lumen area. Venous blood volume is strongly affected by the subject position, since the hydrostatic pressure in the patient's body can cause pooling of the blood in highly distensible veins.
Yet another criticism of prior methods of making non-invasive blood pressure measurements concerns the manner in which the PWV measurements are made. Typically, any PWV measurement that is made between widely separated sites is measuring the PWV of a collection of branches of the arterial tree further complicating the relationship between PWV and blood pressure.
U.S. patent application Ser. No. 10/749,181, commonly assigned with the present application and incorporated herein by reference, teaches a method that utilizes pulse wave velocity to create a continuous, non-invasive blood pressure measurement. The '181 application teaches a method of measuring the pulse wave velocity within a patient and a method of relating such pulse transit time to blood pressure. Although the '181 application teaches a method of accurately measuring the PWV, the relationship to blood pressure can be criticized using some of the same grounds set forth above.
Thus, there is a desire to provide a more accurate, continuous, non-invasive blood pressure measurement technique and method.